> Millenials talk about not wanting children because they don't want to subject a new generation to this absolute shitshow of a planet

They often list overpopulation as a reason. But yes, it's not the only reason they give.

As an antidote to this kind of mindless negativity, just consider whether any individual human is happy to have been born. It seems very clear to me that most people enjoy being alive and so having children is moral on utilitarian grounds.

Bad argument. You could have had even more children, is the fact you didn't immoral? What kind of monster are you?